My fish started getting sick about a week ago, when two of the guppies started showing signs of what i thought to be finrot.
I separated the them into my QT (5 gal fluval chi). Both the guppies had clamped fins, were pale colored and showed difficulty swimming, almost as if the tail was being paralyzed. When i shined light on them I could see a white haze over ones body but the other one did not.

Initially I gave them salt baths in the QT and observed them for any changes.
I also added garlic juice in the tank. They didnt seem to show any signs of recovery. Since they were having difficulty swimming i transferred them into a smaller 1 gallon fish bowl where I tried to treat them with Melafix and Quick Cure (contains Formalin and Malachite Green). But they were dead by morning. I should mention that both QT tanks were properly aerated as I've heard medication may deprive fish of oxygen.

The very next day one of the swordtails was paralyzed. He had been moving absolutely fine the day prior. He was being blown around by the current in the 55 gal tank. I placed him in the QT tank with the guppies. He appeared fine for the first hour, then suddenly began violently darting about. The conditions in the QT tank were ideal. One guppy died, followed by this swordtail, and finally the other guppy.

Two days after their deaths, another swordtail showed signs of distress. He was reclusive, still bright colored and able to swim. So I took him out and put him in the QT (I sterilized it using baking soda and vinegar and thoroughly rinsed it with tap water). I took great care that the QT tank had ideal conditions once again, only this time without salt. He seemed much happier and swam around more in the 5 gallon. I suspected the remaining swordtail male had been bullying him so I thought I had found a solution. After seeing him appear to get better over the period of four hours, I went to bed. I saw him dead in the QT this morning. His body, like the previous three fish, was at the bottom of the tank.

All the sick fish did not eat after a while.

I would appreciate if anyone could shed some light on what could possibly be the issue. I have done quite a bit of research and I can't seem to find symptoms that match what has happened.

The rest of my fish are very bright colored and active, and behaving normally.

Some pictures are attached. They are somewhat graphic in nature so my apologies.

Ph may be a bit low for guppies and swords. They do better in a higher ph and harder water.
Thye also seem to do a bit better in cooler water low to mid 70's. Neons on the other hand do better in the lower ph range and softer water, higher temp as you have.

The abdomen on the fish look a bit recessed to me that and the fact multiple fish are going through the same thing could be a indication that you have chronic wasting syndrome in the tank. This is a form of Tuberculosis.

Treatment and Control:
Kanamycin or other gram negative aquarium treatment.

Along with Vitamin B-6 for 30 days is the most effective treatment.

All of the fish should be quarantined during treatment time and do not add any more fish until it is eliminated. Liquid baby vitamins found at your local pharmacy are a good source of vitamin B-6. One drop per every 5 gallons of aquarium water is sufficient. Replace the vitamins according to how much water is changed in the tank during treatment time.

Overcrowding and poor water quality are usually the cause of this disease.
And caution should always be taken here as in any case, but in this particular case if this is what they actually have this disease may be spread to humans.

The abdomen on the fish look a bit recessed to me that and the fact multiple fish are going through the same thing could be a indication that you have chronic wasting syndrome in the tank. This is a form of Tuberculosis.

Treatment and Control:
Kanamycin or other gram negative aquarium treatment.

Along with Vitamin B-6 for 30 days is the most effective treatment.

All of the fish should be quarantined during treatment time and do not add any more fish until it is eliminated. Liquid baby vitamins found at your local pharmacy are a good source of vitamin B-6. One drop per every 5 gallons of aquarium water is sufficient. Replace the vitamins according to how much water is changed in the tank during treatment time.

Overcrowding and poor water quality are usually the cause of this disease.
And caution should always be taken here as in any case, but in this particular case if this is what they actually have this disease may be spread to humans.

Thanks for the response guys. fish_doc, I will get the medicine and treat as advised.

Please read below as it may provide further clues for this investigation.

Lost the last swordtail in the same way as the other two. Now the fish in the tank are: 13 neon tetras, 4 guppies, 3 clown plecos, one apple snail and one assassin snail.

I bought a master test kit to accurately measure ammonia nitrites, nitrates and pH. The levels are ideal just as I mentioned earlier.

One thing I think i should share is that since setting up the tank, there has been a white mold like growth on the sand substrate. Initially it grew overnight and covered a large area in the tank. I took my gravel vac and tried to get rid of this growth. Also stirred it with my finger in the hopes of clearing it out. Since then, it has been growing in smaller patches or along the edges of the tank. I am not sure what it is but please watch the video and see if you can identify it. I am not sure but is it possible that this growth might be related to the disease that the killed the fish?

Also I dont think I have been overfeeding the fish. During the time when they were getting sick I fed them once every three days. And right now since the remaining fish seem to be healthy, I have resumed feeding them once a day.